I'm running Dynomax on two cars. The first is a 2.5 inch TTI system from stock 440 manifolds and H pipe. It has the dreaded drone at 45MPH or around 2200 RPMS. I get by that speed quickly but should fix it. The second is a total TTI system on a 440, 2" headers into a 2.5 inch system H pipe. This one sounds great at all speeds and have lots of people commenting on how good it sounds. I also run the OTP electric cut-outs on this one, they're a blast! I terrorize the cheby neighbor all the time!